title: Mutation of H63 and its catalytic affect on the methionine aminopeptidase from Escherichia coli.
abstract: In order to gain insight into the mechanistic role of a flexible exterior loop near the active site, made up of Y62, H63, G64, and Y65, that has been proposed to play an important role in substrate binding and recognition in the methionyl aminopeptidase from Escherichia coli (EcMetAP-I), the H63A enzyme was prepared. Mutation of H63 to alanine does not affect the ability of the enzyme to bind divalent metal ions. The specific activity of H63A EcMetAP-I was determined using four different substrates of varying lengths, namely, l-Met-p-NA, MAS, MGMM and MSSHRWDW. For the smallest/shortest substrate (l-Met-p-NA) the specific activity decreased nearly seven fold but as the peptide length increased, the specific activity also increased and became comparable to WT EcMetAP-I. This decrease in specific activity is primarily due to a decrease in the observed k(cat) values, which decreases nearly sixty-fold for l-Met-p-NA while only a four-fold decrease is observed for the tri- and tetra-peptide substrates. Interestingly, no change in k(cat) was observed when the octa-peptide MSSHRWDW was used as a substrate. These data suggest that H63 affects the hydrolysis of small peptide substrates whereas large peptides can overcome the observed loss in binding energy, as predicted from K(m) values, by additional hydrophilic and hydrophobic interactions.
authors.affiliation: Department of Chemistry, Boston University, Boston, MA 02215-2521, USA.
authors.forename: Sanghamitra
authors.initials: S
authors.lastname: Mitra
authors.forename: Brian
authors.initials: B
authors.lastname: Bennett
authors.forename: Richard C
authors.initials: RC
authors.lastname: Holz
mesh_terms: D000409:Alanine / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D000626:Aminopeptidases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D055162:Biocatalysis; D003035:Cobalt / Q000737:chemistry / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology*; D006639:Histidine / Q000235:genetics* / Q000378:metabolism*; D006868:Hydrolysis; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D063208:Methionyl Aminopeptidases; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D013379:Substrate Specificity
chemical_list: D003035:Cobalt; D006639:Histidine; D000626:Aminopeptidases; D063208:Methionyl Aminopeptidases; D000409:Alanine